The present invention relates generally to determining the cell capacities in a multi-cell battery, and more particularly to systems and methods determining cell capacity values based on the state of charge of the battery.
Automotive technology is rapidly expanding in the area of finding alternatives to using gasoline as the primary source of energy in vehicle propulsion systems. Many of these advances utilize either a hybrid mechanical-electrical system that recaptures some of the mechanical energy from the combustion engine as stored electrical energy, or a fully-electric propulsion system, which eliminates the need for an internal combustion engine entirely. With these advancements, the storage and management of electrical energy in vehicles has become of particular importance.
State of charge (SOC) is a commonly-used measure of the amount of charge available in a battery relative to the battery's capacity. In automotive applications that use fully electric or hybrid-electric propulsion systems, SOC measurements provide a useful indication of the amount of energy available to propel the vehicle. Similar to the information provided by a fuel gauge, an SOC measurement can provide a driver of an electric vehicle with an indication of how long the vehicle may travel before running out of energy.
The actual capacity of the battery is another important metric that denotes the overall amount of charge that can be stored in the battery. Typically, a battery is rated for capacity at its time of manufacture. However, as a battery ages, its capacity also decreases. In automotive applications, determination of the battery's actual capacity becomes extremely important because of its effect on SOC measurements. Where a battery's SOC measurement is somewhat analogous to how “full” a conventional fuel tank is in relation to its total volume (e.g., its capacity), batteries differ from conventional fuel tanks because their total capacities decrease over time. For example, a vehicle battery may only have 80% of its original capacity as it ages. Therefore, the actual capacity of a battery may be used to evaluate the overall condition and performance of the battery, in addition to adjusting its SOC estimations.
The amount of information available about the condition and performance of a vehicle battery system is limited when traditional battery capacity estimation techniques are used. Vehicle battery packs usually contain multiple modules which, in turn, include multiple battery cells. However, traditional techniques only estimate the battery capacity at the pack level and/or include numerous sources of error. Such techniques provide only limited information about the cells in the aggregate and fail to identify defective cells that may be underperforming. Inclusion of these cells into a pack-level capacity estimate may also skew the overall results for the pack.